The present invention relates to a flexible printed wiring board and a method of manufacturing it.
Conventionally, flexible printed wiring boards are employed in various different fields, and their shapes are various.
For example, a typical flexible printed wiring board for use in driving a liquid panel may be of the T-shaped type as shown in FIG. 6. The terminals on the BB side are formed with relatively wide wiring pitch so that they can pass signals to the IC in a bundled fashion; on the other hand, the terminals on the AA side are formed with comparatively narrow wiring pitch so that they can be connected to a miniature semiconductor module for drive purposes.
When manufacturing flexible printed wiring boards of T-shape as in FIG. 6, a plurality of T-shaped flexible printed wiring boards per unit area of a laminated sheet for flexible printed wiring purposes obtained by forming insulating layers on conductive layers are created, and this sheet is cut to produce the individual wiring boards. Specifically, as shown in FIG. 7, T-shaped flexible printed wiring boards as shown in FIG. 6 (size: a=160 mm/b=17 mm/c=30 mm/d=20 mm/e=25 mm/f=115 mm) are created by the additive or subtractive method, utilizing a photolithographic technique or the like in a flexible printed wiring laminated sheet 71 of size: length 250 mm, width 200 mm.
However, when T-shaped flexible printed wiring boards 72 were created in a laminated sheet 71 for flexible printed wiring as shown in FIG. 7, about 58% of the laminated sheet 71 for flexible printed wiring was discarded, presenting an obstacle to a reduction in manufacturing costs. This problem becomes more severe as the height (depth) of irregularities of the external shape of the flexible printed wiring board and or its complexity increase.
In view of the above problems of the prior art, an object of the present invention is to confer on a flexible printed wiring board a structure such that as many as possible flexible printed wiring boards can be obtained from a laminated sheet for flexible printed wiring of a prescribed size.
The present inventors perfected the present invention by discovering that: (1) when creating flexible printed wiring boards in a laminated sheet for flexible printed wiring, the amount of laminated sheet discarded could be reduced by creating the flexible printed wiring boards divided into at least two parts; (2) when producing a single flexible printed wiring board by joining at the two parts, it is preferable for connection reliability that bump connection is achieved when the front face side of one part is connected with the back side of the other part; (3) if bumps are employed that are obtained by forming holes by chemical etching utilizing a photolithographic method in the insulating layer of the laminated sheet for flexible printed wiring, then filling these holes with metal plugs by an electrolytic plating method and further growing to metal bumps, it is not necessary to make the positional alignment accuracy excessively high when forming the holes, thereby making it possible to introduce the metal bumps with low cost.
Specifically, the present invention provides a flexible printed wiring board comprising a first flexible printed wiring part having metal bumps and a second flexible printed wiring part having connection pads, the metal bumps and the connection pads being connected to each other, wherein the first flexible printed wiring part comprises a conductive layer and an insulating layer adjacent thereto; holes are provided in the insulating layer so as to reach the conductive layer; metal plugs are formed in the above-mentioned holes by an electrolytic plating method; and the tips of the metal plugs constitute the metal bumps that project from the insulating layer.
Also, the present invention provides a method of manufacturing such a flexible printed wiring board, comprising:
(a) a step of creating first flexible printed wiring parts and/or second flexible printed wiring parts in a laminated sheet for flexible printed wiring composed of a conductive layer and an insulating layer formed adjacent thereto, such that as many as possible first flexible printed wiring parts and/or second flexible printed wiring parts can be obtained per unit area of the laminated sheet;
in which the metal bumps of the first flexible printed wiring parts are created by forming holes in the insulating layer adjacent to the conductive layer, so as to reach the conductive layer, by chemical etching using a photolithographic method, then, while forming metal plugs in the holes of the insulating layer by an electrolytic plating method in which the conductive layer is used as the cathode, further continuously growing these metal plugs by an electrolytic plating method so that the tips thereof project from the surface of the insulating layer;
(b) a step of obtaining the first flexible printed wiring parts and second flexible printed wiring parts from the laminated sheet for flexible printed wiring by cutting the same; and
(c) a step of sticking together the first flexible printed wiring parts and the second flexible printed wiring parts that have thus been obtained while ensuring conduction between the metal bumps of the first flexible printed wiring parts and the connection pads of the second flexible printed wiring parts.